Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. The basic physical structure of a fuel cell includes a porous anode, a porous cathode, and an electrolyte layer disposed between the porous anode and the porous cathode. The electrolyte layer is in immediate physical contact with both the anode and the cathode. A basic schematic diagram of a fuel cell is shown in FIG. 1. As illustrated therein, in a conventional fuel cell, fuel is fed continuously to the porous anode and an oxidant is fed continuously to the porous cathode.
Various fuels and oxidants are known in the art. As one example, the fuel may include hydrogen gas and the oxidant may include oxygen from air. In such a fuel cell, the reaction occurring at the anode is shown in Reaction [1] below, the reaction occurring at the cathode is shown in Reaction [2] below, and the overall reaction is shown in Reaction [3] below.H2+O2−→H2O+2e−  [1]½O2+2e−→O2−  [2]H2+½O2→H2O  [3]
The negatively charged oxygen ions generated by the cathode migrate through the electrolyte layer from the cathode to the anode, while the electrons travel through the external circuit from the anode to the cathode.
A background description of fuel cells can be found in Chapters 1 and 2 of the Fuel Cell Handbook, Seventh Edition, which was prepared by EG&G Technical Services, Inc. for the United States Department of Energy and published in November of 2004, the entire contents of which Chapters are incorporated herein in their entirety by this reference.
One particular type of fuel cell is the solid oxide fuel cell (SOFC). In a conventional solid oxide fuel cell, the electrolyte layer includes a solid, non-porous metal oxide, typically perovskites, such as yttria-stabilized zirconia (i.e., Y2O3-stabilized ZrO2, which is often abbreviated “YSZ”). The anode may be a ceramic-metal composite material (i.e., a “cermet” material), such as a Ni—ZrO2 cermet material or a Ni—YSZ cermet material, and the cathode may be a lanthanum-based perovskite material, such as Sr-doped LaMnO3. A solid oxide fuel cell is conventionally operated at temperatures between about 600° C. and about 1000° C., at which the electrolyte layer exhibits ionic conduction (typically conduction of oxygen ions). Solid oxide fuel cells can be used with a wide range of fuels, in addition to hydrogen gas, including hydrocarbon fuels.
One particular type of solid oxide fuel cell is the tubular solid oxide fuel cell. As the name implies, the tubular solid oxide fuel cell has a tubular structure. A cross-section of a typical single tubular solid oxide fuel cell 10 is shown in FIG. 2. As shown therein, the tubular solid oxide fuel cell 10 includes a generally cylindrical air electrode 12, a generally cylindrical electrolyte layer 14 disposed concentrically over the air electrode 12, and a generally cylindrical fuel electrode 16 disposed concentrically over the electrolyte layer 14. The air electrode 12 may be the cathode and the fuel electrode 16 may be the anode. In this configuration, the fuel may be passed over the external surface 17 of the fuel electrode 16, and air may be introduced into the interior of the tubular solid oxide fuel cell 10 so as to contact the interior surface 13 of the air electrode 12. An air supply tube 20 may be used to introduce the air into the interior of the tubular solid oxide fuel cell 10. As shown in FIG. 2, the electrolyte layer 14 and the fuel electrode 16 may extend only partially around the air electrode 12 to allow an interconnect structure 24 and contact material 26 to be provided on an exposed region of the air electrode 12. For example, the interconnect structure 24 may include a conductive ceramic (such as lanthanum and yttrium chromites doped with elements such as Mg, Sr, Ca, and/or Co) in a fuel cell 10 operating at relatively high temperatures, or the interconnect structure 24 may include a conductive metal or metal alloy in a fuel cell 10 operating at relatively low temperatures. The contact material 26 may include nickel felt, for example. The interconnect structure 24 and the contact material 26 may extend substantially along the length of the tubular solid oxide cell 10, and may be used to interconnect the tubular solid oxide fuel cell 10 with another substantially similar tubular solid oxide fuel cell 10, as described in further detail below.
Referring to FIG. 3, a fuel cell system 30 may include a stack of tubular solid oxide fuel cells 10. The stack may include a number of rows and columns of tubular solid oxide fuel cells 10. While the fuel cell system 30 shown in FIG. 3 includes six tubular solid oxide fuel cells 10, fuel cell systems 30 may include tens or even hundreds of individual solid oxide fuel cells 10. As shown in FIG. 3, the fuel electrode 16 (or whichever of the air electrode 12 and the fuel electrode 16 is disposed on the outside of the fuel cell 10) of each fuel cell 10 may be electrically connected to the air electrode 12 of an adjacent fuel cell 10 through the interconnect structure 24 and contact material 26 of the adjacent fuel cell 10. In other words, as the fuel cells 10 are stacked together, electrical contact is established between the fuel electrode 16 and the interconnect structure 24 of an adjacent fuel cell 10 through the contact material 26.
The air electrodes 12 of each fuel cell 10 in one end row 34 in the stack may be electrically connected to a cathode bus 40, and the fuel electrodes 16 of each fuel cell 10 in an opposite end row 36 may be electrically connected to an anode bus 42. In this configuration, each of the fuel cells 10 in each column may be electrically connected in series between the cathode bus 40 and the anode bus 42. Furthermore, the fuel electrodes 16 (or whichever of the air electrode 12 and the fuel electrode 16 is disposed on the outside of the fuel cell 10) of the fuel cells 10 in each row may be electrically connected together using additional contact material 26. In this configuration, each of the fuel cells 10 in each row may be electrically connected in parallel. The operating voltage of a fuel cell system, such as the fuel cell system 30 shown in FIG. 3, may be increased by increasing the number of individual fuel cells 10 electrically connected in series in each column between the cathode bus 40 and the anode bus 42, and the operating current may be increased by increasing the number of individual fuel cells 10 electrically connected in parallel in each row.
FIG. 4 is a schematic diagram further illustrating operation of a conventional tubular solid oxide fuel cell system, such as the fuel cell system 30 shown in FIG. 3. As shown in FIG. 4, each of the tubular solid oxide fuel cells 10 may have an open end 46 and a closed end 48. A majority of the length of each fuel cell 10 may be enclosed in a fuel chamber 60 defined by a housing or container 50. The open end 46 of each fuel cell 10 may extend through a first barrier or plate 52 and a second barrier or plate 54 and into a combustion chamber 64. The first plate 52 and the second plate 54 may define a recirculation chamber 62 therebetween that communicates with the fuel chamber 60 proximate an inlet 61 thereof.
Deliberately imperfect seals may be provided between the outer surface of each fuel cell 10 and each of the first plate 52 and the second plate 54. In such a configuration, at least some unreacted fuel may be allowed to pass between the outer surface of each fuel cell 10 and the first plate 52, into the recirculation chamber 62, and back into the fuel chamber 60. Furthermore, at least some unreacted fuel may be allowed to pass from the recirculation chamber 62, between the exterior surface of each fuel cell 10 and the second plate 54, and into the combustion chamber 64.
An air supply tube 20 may extend into the interior region of each fuel cell 10 for supplying air thereto. Each air supply tube 20 may have an open end 21 positioned proximate the closed end 48 of each respective fuel cell 10. In this configuration, air may be caused to flow through the interior region of each fuel cell 10 from the closed end 48 thereof generally towards the open end 46. As unreacted air exits from the open end 46 of each fuel cell 10 into the combustion chamber 64, the unreacted air may mix with unreacted fuel and caused to combust. The combustion chamber 64 may be positioned proximate the inlet 61 to the fuel chamber 60 such that the heat generated by the combustion of the unreacted air and the unreacted fuel may be used to preheat the fuel entering into the fuel chamber 60, and to facilitate heating of the fuel cells 10 to the operating temperature of the fuel cell system 30.
Fuel cells are closely related to electrolytic cells, and many fuel cells can be operated as an electrolytic cell for performing electrolysis of a substance by replacing the external circuit associated with the fuel cell with a voltage source (such as, for example, a battery), providing a substance to be electrolyzed in contact with the anode and the cathode, and applying a voltage between the anode and the cathode using the voltage source. For example, water may be provided in contact with the anode and the cathode, and a voltage may be applied between the anode and the cathode, which may cause oxygen gas to be formed at the anode and hydrogen gas to be formed at the cathode.